The present invention relates generally hermetic sealing of lasers. The invention relates in particular to a closed-loop purging system for ozone, water vapor, organic vapor and particulate content in an enclosure surrounding an ultrafast laser resonator or an ultraviolet (UV) laser resonator.
Ultrafast lasers are generally regarded as being lasers that deliver output radiation in pulses having a duration of a few hundred femtoseconds or less. One common ultrafast laser is a Ti:sapphire laser, which can be arranged to deliver output radiation at wavelengths between about 700 nanometers (nm) and about 1000 nm. The pulses delivered often have a relatively low energy, for example, tens of millijoules (mJ) to as little as tens of nanojoules (nJ). The short pulse-duration can cause the pulses to have a very high peak power, for example, on the order of gigawatts per square centimeter. (GW/cm 2) in certain locations in a resonator.
The very high peak powers delivered by such lasers can rapidly cause damage to optical components of the lasers, absent measures to inhibit such damage. Laser damage to optical components may be exacerbated by defects on or in optical surfaces of the components. Accordingly, it is not unusual that at least some portion of the optical components of an ultrafast laser are generated by so-called super-polishing techniques which yield surfaces having a surface smoothness of atomic dimensions, for example, about 4 xc3x85ngstrom Units (xc3x85) root-mean-square (RMS) or less. Optical coatings for such super-polished components, reflective coatings in particular, are often deposited by ion-beam sputtering (IBS). IBS is a coating deposition method that can provide coatings having a high degree of chemical perfection and very low defect content. This minimizes absorption and scattering of radiation by the coatings. However, a super-polished, IBS-coated optical component can be as much as about five or more times more expensive than a similar component polished and coated by more conventional methods. Such additional expense can be wasted if the components are later contaminated by particulate matter, condensates, vapors, or the like.
It is not unusual in commercial laser manufacture to assemble lasers in clean-room conditions to minimize particulate deposition on optical components of the lasers. In such a case, it would be usual to place at least the optical resonator of the laser in an enclosure sufficiently sealed to minimize, at least, ingress of particulate contaminants, and preferably also, ingress of contaminants in gaseous or vapor form. Such an enclosure may be purged, before sealing, with filtered dry nitrogen, dry air or the like.
By implementing one or more above-discussed measures during manufacturing and assembly, an ultrafast laser may be operated for a total of as long as several thousand hours before the performance of the laser becomes significantly diminished by laser damage to one or more optical components thereof. It is believed, however, that even if an enclosure could be perfectly hermetically sealed, damage to optical components may result from contamination of optical components by outgassing products of the optical-components, adhesives and the enclosure itself. Outgassing products can be generated while the laser is operating and also while the laser is not operating.
It is believed that the most problematical of the outgassing products are organic vapors, which can be released from material such as adhesives, elastomer seals, and any plastic materials used in the construction of the enclosure. Water vapor may also be released from components of the enclosure or optics therein. The water vapor and the organic vapors can condense directly on surfaces of the optical components. The water vapor and organic vapors together or in combination can react with laser radiation while laser is operating. Products of the reactions can also condense or be deposited on the optical surfaces. These reaction products may include particulate matter such as carbon particles or soot. Most of these reaction products, if condensed or deposited on the optical surfaces can increase the vulnerability of the optical surfaces to damage by the laser radiation. Application Ser. No. 09/901,857, filed Jul. 9, 2001, assigned to the assignee of the present invention and incorporated herein by reference discloses one arrangement for minimizing contamination of optical components of a laser resonator. In this arrangement, a purging system extracts gas from the enclosure and passes the gas through a desiccant, an organic vapor trapping material, and a particulate matter filter, then returns the extracted gas to the enclosure. This arrangement has been found to be effective in removing most reaction products of the interaction of laser radiation with contaminants present in the enclosure as well as the contaminants themselves. In instances where the laser resonator generates ultraviolet laser radiation, however, it has been found that ozone can be generated as a reaction product of the laser radiation with oxygen or one or more contaminants. Ozone generation becomes problematical when the ultraviolet radiation has a wavelength less than 270 nm and becomes more problematical the shorter the wavelength. The level of generated ozone is not significantly reduced by the purging system and has been found to cause some deterioration of resonator components and absorption of ultraviolet radiation resulting in lowered power output.
The present invention is directed to a method of minimizing contamination of optical components of a laser, the components being located in a gaseous atmosphere within an enclosure. The gaseous atmosphere can contain contaminants including water vapor, organic vapor, and suspended particulate matter. These contaminants may be present at some low level, for example, hundreds of parts per billion or less, immediately after the components are placed in the enclosure. The contaminant level can increase with both operational and non-operational time of the laser. In a laser system providing ultraviolet radiation the atmosphere in the enclosure may include ozone generated as a result of interaction between one or more of the contaminants and the ultraviolet radiation.
In one aspect of the present invention, the method comprises extracting gas from the atmosphere within the enclosure. The extracted gas is passed through a catalyst to convert ozone into oxygen, through a first medium selected to reduce the water vapor content of the extracted gas; through a second medium selected to reduce the organic vapor content of the extracted gas; and through a filter selected to reduce the particulate matter content of the extracted gas. After the extracted gas is passed through the catalyst, the first and second media and the filter, it is returned to the enclosure.
The extraction and replacement cycle preferably takes place continuously during operation of the laser such that the water vapor, organic vapor, and particulate matter content of the atmosphere in the enclosure is maintained at a minimum consistent with the selection of the media and the filter.
In another aspect of the invention, in an enclosure for a laser generating radiation at a wavelength of less than 270 nm, ozone generation can cause significant power loss, a problem that cannot be adequately mitigated by filtering and vapor trapping alone, or by pre-purging the enclosure with an inert gas. Accordingly, in such an enclosure, the ozone removal of the present invention is important as much for efficient operation of the laser as for preventing progressive deterioration of laser components.
In yet another aspect of the invention, apparatus for carrying out the method includes a gas conditioning arrangement including the catalyst, the first (a desiccant) medium, the second (a medium for trapping organic vapors) medium, and the filter for trapping particulate matter. The apparatus includes a pump, which is arranged to extract gas from the enclosure and deliver the extracted gas to the gas-conditioning arrangement. The gas conditioning arrangement is configured such that the extracted air is drawn by the pump over the catalyst, then urged by the pump through the desiccant medium, the organic vapor trapping medium, and the filter, and is subsequently returned to the enclosure.
In one preferred embodiment, the apparatus further includes first and second valves. The first and second valves are arranged such that a drying gas may be circulated through the desiccant medium for regenerating the desiccant medium while preventing the drying gas from reaching the enclosure.
Maintaining a low organic vapor content in a laser resonator is particularly important if the laser resonator is an ultrafast laser resonator or a laser resonator arranged to generate ultraviolet laser radiation. The relatively high-energy of ultraviolet laser radiation, multiphoton processes in the case of ultrafast lasers, generating longer wavelength radiation can increase the probability of reactions between the laser radiation and the organic vapors or their condensates. As noted above, products of these reactions, including particulate matter, can lead to unstable operation of the laser, or accelerated damage to optical components of the laser resonator.